Apparatus is known to the prior art for measuring the electrical impedance of particulate material in bulk form, such as wood chips or grain. The measurements may be made on a continuous basis, in which the material is continuously flowing past an electrode forming part of the apparatus, or on a sampling basis, in which a sample of the material is placed in a container including or forming part of the electrode.
In those apparatus which operate on a sampling basis, the apparatus may include a sample box for receiving and retaining a sample of the particulate material during measurement. An example of such a sample box is disclosed in U.S. Pat. No. 4,107,599, issued Aug. 15, 1978 to Fritz K. Preikschat, and entitled ELECTRODE FOR AN IMPEDANCE MEASURING APPARATUS. The sample box includes a grounded electrode portion which is shaped as a box having a substantially rectangular cross section. An active center electrode, typically comprising a metallic plate, is disposed in the interior of the sample box and in parallel, spaced relationship to the side walls thereof so that a uniform electrical field may be created within the sample box. The sample box has an inlet and an outlet which, together with the grounded electrode portion and the active center electrode, define a constant volume into which a sample of particulate material may be placed and retained for measurement. A temperature sensor is located within the sample box for providing a signal related to the temperature of the sample, and a weight sensor is operatively associated with the sample box so as to provide a signal related to the bulk density (weight per unit volume) of the sample.
As disclosed in U.S. Pat. No. 4,181,881, IMPROVED ELECTRICAL IMPEDANCE MEASURING APPARATUS FOR PROVIDING SEPARATE MEASUREMENTS OF THE CONDUCTIVITY AND DIELECTRIC COEFFICIENT OF VARIOUS MATERIALS, issued on Jan. 1, 1980 to Fritz K. Preikschat, a signal generator provides a high-frequency test signal which is coupled to a bridge circuit located in proximity to the sample box and interconnected with the active, center electrode and the grounded electrode portion thereof. As a result, a high-frequency electrical field is generated within the sample box, whereupon the bridge circuit provides a bridge output signal whose frequency is identical to that of the test signal. The phase of the bridge output signal, relative to that of the test signal, and the amplitude thereof are related to the electrical admittance of the sample. The signal generator also provides a reference signal whose frequency is identical to that of the test signal and whose phase is successively shifted so as to be in-phase and 90.degree. out-of-phase with the test signal. The reference signal is supplied to a double-balanced mixer which also receives the bridge output signal. A time-multiplexed output signal from the mixer includes components successively related to the conductivity and to the dielectric coefficient of the sample, and is coupled to further signal processing circuitry which functions to compensate the time-multiplexed output signal for variations in temperature and bulk density of the sample in accordance with signals from the temperature sensor and the weight sensor. The time-multiplexed output signal is then demultiplexed to provide separate conductivity and dielectric coefficient signals which may be displayed, recorded, or supplied to a digital computer for data processing.
As is well known, the conductivity and dielectric coefficient signals are each related to the moisture content of the sample. However, the dielectric coefficient signal is typically used for determination of moisture content and may be further compensated in accordance with the value of the conductivity signal (see, for example, U.S. Pat. No. 4,174,498, APPARATUS AND METHOD FOR PROVIDING SEPARATE CONDUCTIVITY, DIELECTRIC COEFFICIENT, AND MOISTURE MEASUREMENTS OF PARTICULATE MATERIAL, issued Nov. 13, 1979 to Fritz K. Preikschat).
In measuring the electrical impedance of certain particulate material such as wood chips, it has been found that the conductivity and dielectric coefficient signals undergo substantial changes as the moisture in a sample changes between frozen and unfrozen states. For example, the value of each signal obtained when the moisture in a sample is entirely frozen may be less than half of that obtained when the moisture in the sample is completely unfrozen. Therefore, the determination of moisture content by measuring of electrical impedance of a sample is subject to significant error when the sample contains moisture in a frozen state.
Another method for the determination of moisture content of particulate material such as wood chips involves measurement of the average oven dry bulk density of a particular source of wood chips from which the sample has been taken (e.g., the average bulk density when all free moisture has been removed). The oven dry bulk density is then divided by the bulk density of the sample (e.g., that represented by the output signal from the weight sensor) with the resultant quotient comprising the oven dry, or fiber content, percentage of the sample (where moisture percentage plus fiber content percentage-100). It has also been found that the bulk density of a sample changes as the moisture in the sample changes between frozen and unfrozen states. For example, the bulk density of a sample may decrease by about 6% as the moisture within the sample changes from completely unfrozen to completely frozen. Therefore, the determination of moisture content by measuring the bulk density of a sample is also subject to significant error when the sample contains moisture in a frozen state.
The determination of moisture content is further complicated by the fact that the sample may contain moisture in unknown amounts in both frozen and unfrozen states. If is therefore difficult to determine if the sample is frozen, and if so, to determine the degree of frozenness thereof. Although the temperature of the sample may seem to be a promising indicator of the presence of frozen moisture, it has been found that the output signal from the temperature sensor of the impedance measuring apparatus may represent a measured sample temperature as high as 5.degree. C. even though a substantial portion of the moisture is still in a frozen state. Further, it has been found that the measured sample temperature is not related in a predictable manner to the degree of frozenness of the sample. Since it has not been known how to accurately determine the presence or amount of frozen moisture in a sample, it has not been possible to provide compensation of moisture content determinations made using either the dielectric coefficient or bulk density output signals from the impedance measuring apparatus.
Accordingly, it has been thought desirable to completely thaw the wood chips before any measurement of moisture content is made. In order to accomplish such thawing, various thermal and RF heating methods have been proposed. Because of the low thermal conductivity of wood chips, most heating methods are impractical. For wood chips having a typical thickness of one inch, a time period up to ten minutes is required to conduct enough heat into the interior of each chip to convert the moisture to its unfrozen state, assuming ideal conditions with a 100.degree. C. temperature gradient between the exterior and interior of each chip. In addition to the long thawing times required, heating methods oftentimes appreciably affect the moisture content of the sample inasmuch as a certain portion of the moisture is converted during thawing from its unfrozen to its vaporized state. Accordingly, thawing of wood chips prior to moisture content measurement has not proved to be satisfactory.
It is therefore an object of this invention to provide an improved method and apparatus for determining the moisture content of a sample of particulate material such as wood chips.
It is a further object of this invention to provide such a method and apparatus which does not require thawing of the sample prior to moisture content measurement.
It is yet a further object of this invention to provide such a method and apparatus which provide accurate measurement of moisture content, notwithstanding the fact that a portion of the moisture within the sample may be in a frozen state.
It is another object of this invention to provide a method and apparatus which utilizes a plurality of computational techniques for determining moisture content of a sample of particulate material, and which is operative to select one of these techniques in response to various measured parameters of the sample.
It is still another object of this invention to provide a method and apparatus for determining the moisture content of a sample of particulate material which is particularly adapted for use with a sample box and an impedance measuring apparatus of the type disclosed in the aforementioned U.S. Pat. Nos. 4,107,599 and 4,181,881, and which is preferably embodied in a programmed microprocessor consisting of readily-available, integrated circuit chips.